1. Field of the Invention
The present invention relates generally to apparatus and methods for reducing electrical field emissions, and more specifically to an electrostatic shield and method for reducing nearfield alternating electrical field emissions in a CRT display.
2. Brief History of the Art
Cathode ray tube (CRT) displays are used in conjunction with a wide variety of electrical systems, and are most commonly associated with ordinary television sets and computer monitors. These displays normally employ a deflection yoke for controlling an electron beam which produces a visible image on the CRT screen. The deflection yoke generates a complex alternating electromagnetic field, a portion of which emanates towards and through the screen, eventually reaching the device operator. This electromagnetic field includes an AC magnetic component, used to control the deflection angle of the electron beam, and an AC electric component.
During the past several years major concerns have been raised by various groups regarding potential health hazards inherent in devices which generate electromagnetic fields. Although there are presently no U.S. government regulatory standards defining harmful versus non-harmful levels of electromagnetic field exposure, several international communities have discussed potential guidelines. Increased public awareness of these potential hazards has also surfaced, and has led to increasing numbers of products designed to limit the amount of electromagnetic field exposure an operator experiences while using a particular device.
Illustrative of such a design, computer monitors normally include metal shielding which substantially covers the monitor chassis. It is a well known physical principle that the field lines of an AC electric field will terminate on any surface which has a zero or near zero potential with reference to the AC source. Thus, the grounded metal shielding covering the monitor chassis effectively terminates the AC electric field inside the chassis wherever the shielding is placed. Although this reduces the resultant AC electric field on the outside of the monitor wherever the shielding is placed, metal shielding of the magnitude used to cover the chassis cannot be placed in front of the CRT screen without seriously degrading monitor performance. Unfortunately, this is precisely the direction of AC electric field line propagation which affects an operator the most.
Existing countermeasures for reducing the frontally directed AC electric field include placing a conductive screen or thin metal film over the CRT face. The screen or film must then be electrically connected to chassis ground. Although these countermeasures help in reducing overall field emissions, there are several serious inherent problems which limit their effectiveness. These problems include degradation of front of screen CRT performance, mechanical form and fit problems and high associated design and production costs. Especially in today's competitive display market, screen performance in a CRT and overall product cost are not features to be compromised. Thus, existing solutions to the AC electric field emission problems are inadequate.
As will be discussed in detail below, the present invention provides an improved electrostatic shield and method for reducing alternating electric field emissions in a CRT display. Because the shield is implemented in the display in non-interfering relationship to the electron beam of the CRT, the present invention provides excellent reduction in the AC electric field emission through the CRT screen, while producing no front of screen performance degradation.